Feline Leukaemia Virus

Also known as FeLV

FeLV Electron Micrograph Public Health Image Library Image #5610

Introduction

Kinetics of FeLV - Copyright Dr Brian Catchpole BVetMed PhD MRCVS

• Oncogenic retrovirus
• Causes neoplasia (lymphoma), myelosuppression (anaemia) and immunosuppression (of T cells)
• 2 strains:
  • FeLV-A
    • Natural strain
  • FeLV-B
    • Formed through FeLV-A recombining with endogenous retroviral sequences in the feline genome
    • Increases the risks of lymphoma
  • FeLV-C
    • Formed from the spontaneous mutation of FeLV-A
    • Is more myelosuppressive
• Virus replicates in the oropharyngeal lymphoid tissue causing a viraemia (virus circulating in the bloodstream) which then spreads to the systemic lymphoid tissue
• Shed in saliva
• Passed by oronasal route, e.g. mutual grooming
• Kittens between 6 weeks and 6 months are most susceptible
• 60% of cats will become immune to the disease and recover
• Cats that are persistently viraemic will progress to develop FeLV-associated diseases
• Some cats will become viraemic again if treated with corticosteroids or stressed if the infection lies dormant in the bone marrow

Pathogenesis

FeLV is the "disease of friends": transmission usually occurs through mutual grooming. From the oropharynx, the virus spreads to most tissues in the body to replicate, notably:

• Bone marrow
• Thymus
• Salivary glands
• Reproductive tract

Most kittens but only 30% of adults become viremic for life without producing antibody. The condition progresses in various forms:

• 20% of viremic cats die of tumors
• 30% of viremic cats die of FeLV-associated disease
• 80% die within three years of exposure

30% of adults exposed become latently infected and can become viremic when immunosuppressed. 40% of exposed adults remain healthy and develop Ab and CD8+ T cells after clearing the virus, without becoming reinfected or silent carriers.

Clinical Signs

• Leukemia - a neoplastic increase in blood cell numbers - usually white blood cells
• Multicentric lymphosarcoma: B or T cell tumors, which may be palpable as enlarged lymph nodes (particularly mesenteric)
• Thymic lymphosarcoma: T cell tumors, with only the thymus enlarged -this may result in dyspnoea and can be confirmed by radiography
• Alimentary lymphosarcoma: B cell tumors of the Peyer's patches

FeLV-associated disease:

• Immunodepression causing secondary disease
• Reproductive failure: FeLV crosses the placenta, causing fetal resorption or viremic kittens with thymic aplasia

Epidemiology

Vertical transmission of FeLV-A occurs from mother to kittens either via placenta, grooming, or milk. Horizontal transmission occurs via saliva during mutual grooming. FeLV is of particular concern for intensively bred cats because of close living conditions shared with other cats.

Recovery is linked to age and the presence of maternal antibody

Antigenicity

Viral subgroups have been categorised based on the different abnormalities detected in the host cellular envelope protein gp70. This protein interacts with a cellular receptor, and can prevent already infected cells from being targeted by further viral organisms.

There are 3 antigenic subgroups which are defined by the gp70 regulated viral response:

• Group A is transmitted between cats and is monotypic: one vaccine covers all isolates. Infection with this viral subgroup leads to lymphosarcoma.
• Group B is recombinant with transmissible FeLV-A; infection with viruses of this subgroup increases the chance of developing thymic tumors.
• Group C is a mutant of subgroup A. Isolates are rare, and occur as A+C mixtures, leading to an increased chance of developing anemia.

Diagnosis

• FeLV should be suspect in any cat with recurrent bacterial infections, anemia or weight loss. Diagnostic tests available for cinfirmation of disease sttus includes:
• ELISA for FeLV antigens (capsid protein p27 or envelope protein gp70)
• Immunochromatography is now preferred as ELISA testing can give false positives
• Virus isolation from heparinised blood can be performed to confirm a positive diagnosis
• Immunofluorescence
• PCR
• Rapid-Immuno-Migration
• Western Blot

Infection Control

Antigen positive cats with significant symptoms require euthanasia. Healthy positive cats should have the diagnosis confirmed by a second alternate testing method.

Vaccination

Ideally, this should take place once antigen-negative status has been determined by laboratory testing. Leukogen is a subunit vaccine (using envelope protein gp70) produced in E. coli mixed with Quill-A and alhydrogel. Others include inactivated virus and canarypox recombinants, but all MUST include FeLV-A.

Treatment

Where treatment is undertaken, anti-retroviral therapy is indicated with antibiotics to control secondary infections. Prevention is a key element in the treatment of FeLV overall.